Lyman-Werner photon

(ultraviolet photon with proper energy to excite molecular hydrogen)

A Lyman-Werner photon is an ultravioletphoton
in the H2Lyman band (aka Werner band, 11.2eV-13.6eV),
which can be absorbed by an H2 molecule,
pushing the molecule to an excited state,
possibility subsequently resulting in the molecule's dissociation,
i.e., splitting into atomic hydrogen.
In molecular clouds, this is the most common
means of photodissociation.